In the telecommunications field, particularly in long-distance networks, long-distance network providers continually strive to increase the traffic carrying capacity of their transmission medium. For example, since fiber optic cables have increased bandwidth over known twisted pair or copper wire cables, fiber optic cables are used increasingly for connecting network stations and other network elements. As a result, a greater number of stations or network elements can be connected over a fewer number of fiber optic cables, as opposed to prior cables. In other words, each fiber optic cable can handle numerous trunks, as opposed to prior cables.
Additional channels can be provided over fiber optic or other cables by a digitizing and multiplexing signals transmitted over such cables. For example, a single T1 trunk can carry 24 DS-0 channels. Each of the 24 channels are multiplexed to provide a continuous series of 8 bit bytes for each channel. Voice is digitized or pulse code modulated (PCM) under the known Mu-law standard employed in Japan and North America. Under the Mu-law standard, a PCM encoding algorithm digitizes each sample into 8 bits, thus providing a 64K transmission rate (the standard rate for encoding voices is 8K samples/second). A sample typically consists of a sign bit, a 3 bit segment specifying a national logarithmic range, and a 4 bit step offset into the range. All bits of the sample are typically inverted before transmission.
To properly transmit a digital signal between transmitting and receiving nodes, alternate mark inversion (AMI) is employed. AMI is a line coding format in T-1 or DS-1 transmission systems whereby successive ones ("marks") are alternately inverted (i.e., sent with polarity opposite that of the preceding mark). If long strings of zeros are transmitted, AMI fails to provide adequate synchronization between transmitting and receiving nodes in a network. Therefore, telephony equipment that uses AMI prohibits any string of eight consecutive zeros from being transmitted on a per channel basis.
Typically, every sixth frame transmitted on a T-1 trunk includes signaling information (e.g., information indicating that a given channel is still off hook, etc.). Many telecommunications network providers are striving to provide 64 kilobit "clear channels," so as to provide 100% data transmission for subscribers to the network (e.g., ISDN trunks). Such clear channels provide 64 kilobits per second bandwidth, with no signaling data transmitted, and are thus ideal for high speed data transmissions. Such data transmissions can generate strings of eight zeros.
An improvement to AMI was developed known as binary eight zeros substitution (B8ZS). Under the B8ZS standard, strings of eight zeros are replaced with a special B8ZS byte by a transmitting node. The receiving node, receiving the special B8ZS byte, converts this byte back into a string of eight zeros. Unfortunately, various nodes and equipment within a network employ either the AMI or the B8ZS line coding standard. AMI/B8ZS line encoding mismatches typically affect data and facsimile messages transmitted over a network. Errors range from slightly distorted facsimile messages to totally indecipherable messages.
If such AMI/B8ZS line coding mismatches are detected, equipment can be replaced or modified to convert to one of the two standards (typically to the B8ZS standard). One known method of detecting such line coding mismatches is to perform a signal channel impulse noise test for a period of five minutes. Adjacent channel activity, together with the impulse noise signal, produces a series of zeros, which can be detected by test apparatus. Thus, this method relies on adjacent channel activity (customer voice or data) to detect mismatches. Even if adjacent channel activity is sufficient to immediately produce a channel mismatch with the impulse noise signal, the test is nevertheless extended for a duration of five minutes to ensure that adjacent channel activity will produce a mismatch if such a mismatch exists along a given trunk.
Such prior method of detecting AMI/B8ZS mismatches takes considerable time to complete when a large digital network must be analyzed. Furthermore, even though a mismatch may be detected, a retest is typically performed by a field technician to confirm that such a mismatch exists. Thus, while the initial test may correctly detect a mismatch, the subsequent retest may in fact fail to detect the mismatch. As a result, the mismatch error may go uncorrected. Moreover, detected mismatches may go uncorrected for months if not years, since a field technician must perform such a retest.
Most digital networks have a mixture of AMI/B8ZS nodes. Due to the drawback of the above test, it is difficult, if not impossible, to totally eliminate such line coding mismatch. Furthermore, human error and changes to the network will, over time, increase the likelihood of such mismatches.